Question

Which of the two following scenarios would be more likely to support life on Earth? 1\. Earth is the same size as at present but has the orbit of Mars. 2\. Earth has the same orbit as at present but is the size of Mars. Explain the reasons you used to support this interpretation. Discuss how the four key characteristics of Earth (liquid water, gravity and atmosphere, life- sustaining gases, magnetic field) described in this section would vary in each scenario.

Short answer

Scenario 1 (Earth's current size with Mars' orbit) is more likely to support life due to maintained atmosphere and magnetic field, despite colder temperatures.

Step-by-step solution

Assessing Current Earth Conditions

Understanding Earth’s current conditions is essential. Earth’s present size and orbit support liquid water on the surface, an atmosphere with life-sustaining gases, stable gravity for maintaining the atmosphere, and a magnetic field protecting it from harmful solar and cosmic radiation.

Scenario 1 - Earth with Mars' Orbit

In this scenario, Earth remains the same size but orbits at the distance of Mars. The increased distance from the Sun would lower temperatures significantly, challenging the maintenance of liquid water. This could lead to most water freezing, affecting life survival. The atmosphere and magnetic field would remain intact due to unchanged size and internal structure, but life-sustaining temperatures could be compromised.

Scenario 2 - Earth the Size of Mars

Here, Earth retains its current orbit but is reduced to the size of Mars. The closer orbit to the Sun supports a warm enough environment for liquid water. However, the reduced size means a weaker gravity, which could not hold a dense, life-sustaining atmosphere as effectively. A smaller planet might also have a weaker magnetic field, risking greater exposure to harmful solar radiation.

Comparing Life-Sustaining Conditions

Between the two scenarios, the presence of liquid water, atmosphere, life-sustaining gases, and a stable magnetic field are crucial. Scenario 1, with Earth’s current size, maintains crucial atmospheric and magnetic protections even with colder temperatures, while Scenario 2 presents challenges with gravity’s inability to support a robust atmosphere due to smaller size.

Key concepts

Liquid Water on Earth

Liquid water is essential for life as we know it on Earth. It serves as a solvent, a medium for chemical reactions, and a habitat for countless organisms. Earth is situated in the "Goldilocks zone," where temperatures allow water to exist in a liquid state. This unique position makes our planet extremely favorable for sustaining life.
In Scenario 1, where Earth orbits at Mars' distance, temperatures would plunge, resulting in extensive ice formation. Liquid water, vital for a thriving ecosystem, might become scarce, as freezing temperatures would prevent large bodies of water from remaining fluid.
In Scenario 2, even though Earth remains in the "Goldilocks zone," its smaller size would not directly impact the presence of liquid water. However, the inability to sustain a thick atmosphere due to weak gravity could lead to surface water evaporating or escaping into space due to insufficient protection.

Gravity and Atmosphere

Gravity on Earth is crucial for retaining a life-supporting atmosphere. It holds the layers of gases around the planet that are necessary for breathing, protection, and maintaining temperature. Earth's gravity allows it to sustain a thick atmosphere, rich with gases like oxygen and nitrogen.
In the Mars orbit scenario (Scenario 1), Earth's gravity would remain unchanged, still capable of preserving a robust atmosphere. The significant concern, however, would be the drop in temperature rather than atmospheric retention.
Conversely, in Scenario 2, Earth being the same size as Mars would result in weaker gravity. This could lead to a much thinner atmosphere, as weaker gravity would struggle to keep gases from escaping into space. The lack of a dense atmosphere could wreak havoc on temperature regulation and weather patterns, making the environment more hostile to life.

Life-Sustaining Gases

The atmosphere is composed of life-sustaining gases that are vital for all known organisms. Oxygen is necessary for the survival of most terrestrial life, while carbon dioxide supports photosynthesis, making Earth's atmosphere unique.
In Scenario 1, Earth's current size allows it to maintain these gases. However, the colder climate could disrupt biological processes that depend on moderate temperatures. Cold conditions might slow or halt the natural cycles that keep these gases in balance.
In Scenario 2, the reduced size of the planet impacts the sustainability of these critical gases due to weaker gravity. A thinner atmosphere could not only mean a loss of oxygen but also a fragility in maintaining the greenhouse gases that trap heat and stabilize temperatures. Such instability would be detrimental to sustaining life.

Earth's Magnetic Field

Earth’s magnetic field is a protective shield against solar and cosmic radiation. It is generated by the movement of molten iron in Earth's core, creating a magnetic shield that deflects harmful particles from the Sun.
In Scenario 1, the size and internal structure remain unchanged, meaning this protective feature stays robust. Despite the colder climate, Earth's magnetic field would still offer protection from radiation.
In contrast, Scenario 2 could face challenges as a smaller Earth may have a weakened magnetic field. The decrease in volume and mass could compromise the dynamo effect in the core, reducing the strength of the magnetic field. Without adequate protection, radiation could strip the atmosphere and damage biological life, creating an inhospitable environment.